Removal of cellulose nitrate subbing layer from safety base films by using an alkalifollowed by a permanganate treatment



Patented Sept. 7, 1954 REMOVAL OF CELLULOSE NITRATE SUB- BING LAYER FROM SAFETY BASE FILMS BY USING AN ALKALI FOLLOWED BY A PERMANGANATE TREATMENT Francis E. Fox, Jr., Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application November 20, 1951,

Serial No. 257,430

5 Claims.

This invention relates to the treatment of film scrap so as to remove cellulose nitrate subbing layers therefrom by treating the scrap first with dilute aqueous alkali followed by a treatment thereof with acidified permanganate.

Ordinarily, photographic film consists of a base of a cellulose ester, one or more subbing layers and a photographic gelatin emulsion thereon. The emulsion layer is removed from film scrap ordinarily by a treatment with hot water whereby the gelatin thereof is softened and the layer can be washed off. The resulting material will then ordinarily consist of a base of a lower fatty acid ester of cellulose, and one or more subbing layers of which often an important constituent is cellulose nitrate. My invention is primarily applicable to the recovery of esters from film scrap in which the film base ester is a cellulose acetate having an acetyl content of at least 42.5% acetyl, cellulose acetate butyrate or cellulose acetate propionate.

One object of my invention is to provide a process of recovering film base cellulose esters which have been subbed with cellulose nitrate layers. Another object of my invention is to provide an inexpensive but effective method of removing a cellulose nitrate subbing layer from cellulose ester sheeting. Other objects will appear herein.

I have found that a sheeting of cellulose acetate propionate, cellulose acetate butyrate or high acetyl cellulose acetate, having a thin layer of a cellulose nitrate composition thereon, may be recovered by first treating the scrap, preferably after being chopped into small particles, with a dilute aqueous solution of alkali, which is then removed, followed by treatment of the scrap with a solution of an alkali metal permanganate which has been acidified. I have found that, where a film base consisting of a high acetyl cellulose acetate, cellulose acetate propionate or cellulose acetate butyrate is subjected to a combination of treatment with alkali and after-treatment with acidified permanganate, the undesirable particles present are readily separated from the film base ester by washing so that the presence of fine insoluble particles is avoided and a recovered product having good properties is obtained.

It has been recognized that permanganate is useful for renovating certain types of safety film scrap; however, in the case of various types of cellulose nitrate subbing layers, such treatment alone has been found to be not particularly effective. Although such layers may be removed by heating the film scrap with alkali, such treatment has been found to result in particles therein which, because of their fineness, have been difficult to separate from the bulk of the film base cellulose ester.

In accordance with my invention, the film base, after removal of the photographic emulsion therefrom, is treated with aqueous alkali at an elevated temperature which is effective in re-v moving the cellulose nitrate subbing layer. This nitrate subbing layer may be a mixture of cellulose nitrate and polyvinyl acetate, cellulose nitrate and gelatin, or cellulose nitrate per se. With any one of the usual types of cellulose nitrate subbing layers on cellulose triacetate, cellulose acetate propionate or cellulose acetate butyrate film base, this alkali treatment, when combined with a second treatment with an aqueous acid solution of permanganate, gives a product which is completely soluble in the solvent which is normally used for its solution.

The alkaline treatment of the film scrap is carried out by heating the scrap containing the nitrate subbing layer all in comminuted con dition with an aqueous solution of an alkali, such as sodium hydroxide or sodium carbonate. The concentration of alkali in the water may be within the range of 0.1255%, the treatment being carried out at a temperature of at least -212 F. In actual practice it is preferred to use a low caustic concentration, such as not more than 4%, as by this treatment the time is not too short, and hence not too difiicult to control, and also less hydrolysis of the film base itself is obtained. The time of treatment varies, both with the type of film base used and with the concentration of alkali employed. For instance, with a given type of film, as the concentration of the alkali increases, the time of treatment can be lessened. However, with two different types of film base, it may take as much as twice the time to remove the nitrate subbing layer from one type as from another type of film base. This, however, will become apparent in the practice of the alkaline treatment.

After the film base has been treated for the desired time with the alkaline liquor, the liquid is removed therefrom and, after washing the alkali therefrom with water, the film base is then treated with an aqueous solution of potassium permanganate and acid. The permanganate and the acid should both be present in the water in a concentration of at least 0.03%, but it is preferred that the acid have a concentration twice that of the permanganate. For instance, if

0.03% permanganate concentration is used, the use of .06% of acid would give a greater speed of treatment by the permanganate solution. The concentration of acid used is limited only by the effect thereof on the film base. For instance, a proportion of acid greatly in excess of 5% would not be desirable in the permanganate treatment. Ordinarily, sulfuric acid would be suitable for use in this operation, but, if desired, some other strong acid, such as nitric, hydrochloric, phosphoric or perchloric could be employed to supply the desired acidity to the mass. As a rule, it may be stated that the more severe the alkaline treatment which is applied to the film base scrap, the greater should be the concentration of permanganate to assure a product having the best properties. However, the upper limit of the alkaline and permanganate concentrations used will, in most cases, be governed by the economics of the film scrap recovery operation.

I have found that ordinarily it is desirable to operate the permanganate step in the treatment of the film scrap at a temperature within the range of 180-212 F. Lower temperatures, however, can be used, but to get a similar effect, either the time of treatment is extended or, better, the concentration of the alkali metal permanganate in the treating bath is increased. After the permanganate treatment it is ordinarily desirable after washing the film scrap to treat it with some solution to discharge any MllOz which may be present, such as by treating the material with an aqueous solution of sodium bisulfite. It is desirable, but not necessary, that this solution have an elevated temperature, such as 100150- F. The solution may be acidified if desired, such as by adding a small amount of sulfuric acid, but it has been found that the desired effect may be obtained without this acid addition. After again washing, the recovered cellulose ester material obtained is then ready for use and can be dissolved in volatile solvents, such as methylene chloride-methanol, and used to form sheeting, filaments, coatings and the like. The plasticizer already present may be sufiicient for the purpose desired, or further plasticizer may be dissolved in the mass.

One type of film base which can be recovered in accordance with my invention is that in which what is ordinarily known as cellulose triacetate is used. That ester ordinarily has an acetyl content within the range of 42.5-43.5% acetyl, although any cellulose acetate having an acetyl content above 42.5% can be recovered by this method. The following examples illustrate my invention:

Example 1 Film scrap, the base of which is cellulose triacetate and which had a subbing layer of cellulose nitrate, was treated with 0.125% sodium hydroxide liquor for minutes at 190 F., the liquid to film support ratio being to 1 by weight. The scrap was then rinsed with water several times and was then treated with a liquor containing 0.03% of potassium permanganate and 0.06% of sulfuric acid, this treatment being for 15 minutes at 190 F. The material was then washed well with a 0.075% solution of sodium bisulfite in water, following which the film scrap was thoroughly rinsed. A cellulose triacetate was obtained by this procedure in which the cellulose nitrate was absent.

Example 2 A film scrap, having a cellulose tri acetate base and a subbing layer of cellulose nitrate, was treated with an aqueous solution of sodium hydroxide of 0.25% strength for 35 minutes at F., the liquid being 20 times the weight of the film scrap. The scrap was then rinsed with water several times and treated with water containing in solution therein 0.045% of potassium permanganate and 0.06% of sulfuric acid, the treatment being for 60 minutes at 190 F. The mass was then thoroughly washed with 0.1% sodium bisulfite liquor and then with water. Cellulose 'triacetate completely free of cellulose nitrate was obtained by this procedure.

Example 3 Film scrap, having a base of cellulose acetate propionate and a subbing layer of cellulose nitrate, was treated with an aqueous solution of sodium hydroxide of 0.125% concentration for 15 minutes at 190 F., the ratio of liquid film scrap being 20:1 by weight. After this treatment, the scrap was rinsed several times with water and was then treated with water containing therein 0.03% of potassium permanganate and 0.06% of sulfuric acid, this treatment being for 15 minutes at 190 F. The material was then washed well with a 0.075% solution of sodium bisulfite in water, after which the film scrap was thoroughly rinsed. Cellulose acetate propionate, free of cellulose nitrate, was obtained by this procedure. The cellulose acetate propionate was found to dissolve in organic solvents to give solutions of excellent clarity.

Example 4 Film scrap, consisting of cellulose acetate butyrate base and a cellulose nitrate subbing layer, was treated with an aqueous solution of sodium hydroxide of 0.25% strength for 40 minutes at 190 F., the liquid being 20 times the weight of the film scrap. The scrap was then rinsed with water several times and was then treated with water containing therein 0.2% of potassium permanganate and 0.2% of sulfuric acid, the treatment being for 45 minutes at 190 F. The mass was then thoroughly washed with 0.25% sodium bisulfite solution and then with water. There was obtained a cellulose acetate butyrate free of cellulose nitrate, which ester dissolved in organic solvents to give solutions of excellent clarity.

In the treatment of film scrap in accordance with my invention it is desirable, with the usual type of stirring equipment, to employ at least 20 parts of treating liquid to l of film scrap to impart good mobility to the mass. However, with some forms of equipment, such as where the film scrap and liquid is agitated by tumbling, less proportions of liquid, such as 10:1 or even 5:1, may be used to obtain satisfactory removal of the nitrate subbing layer from the film scrap.

One feature of my invention is that film scrap, containing various types of nitrate subbing layers, can all be mixed together and treated in accordance with the method which I have described, as my method is effective in removing both nitrate layers, which are readily removable, and nitrate subbing layers, which are more resistant to the effects of aqueous solutions, and therefore require a more effective treatment. Different types of film base containing nitrate subbing layers may also be mixed together and treated in accordance with my invention. However, as

high acetyl cellulose acetate is usually employed with difierent solvents than the mixed esters, such as cellulose acetate propionate and cellulose acetate butyrate, it would ordinarily be preferable to separate cellulose acetate film scrap from that having a mixed ester base prior to treatment as described herein, so as to assure uniformity as to solubility characteristics in the product which is eventually obtained.

I claim:

1. A method of reclaiming film scrap having a lower fatty acid ester of cellulose base and a cellulose nitrate subbing layer thereon, which comprises subjecting the scrap to an aqueous solution of an alkali of 0.125-5% concentration at a temperature of 180-212 F. for a time sufl'icient to loosen the subbing layer and subsequently treating the scrap with water containing at least 0.03% of an alkali metal permanganate and 0.03-5% of a strong mineral acid at an elevated temperature whereby film base material having excellent solubility characteristics is recovered.

2. A method of reclaiming cellulose triacetate film scrap having one or more cellulose nitrate subbing layers thereon, which comprises subjecting the scrap to a 0.1255% solution of an alkali at 180-212 F. for a time sufficient to loosen the cellulose nitrate layer, followed by treating the material thus obtained with a solution of potassium permanganate of at least 0.03% strength acidified with a strong mineral acid at an elevated temperature whereby the resulting product has excellent solubility characteristics.

3. A method of reclaiming cellulose acetate butyrate film scrap having one or more cellulose nitrate subbing layers thereon, which comprises subjecting the scrap to a 0.125-5% solution of an alkali at 212 F. for a time sufiicient to loosen the cellulose nitrate layer, followed by treating the material thus obtained with a solution of potassium permanganate of at least 0.03% strength acidified with a strong mineral acid at an elevated temperature whereby the resulting product has excellent solubility characteristics.

4. A method of reclaiming cellulose acetate propionate film scrap having one or more cellulose nitrate subbing layers thereon, which comprises subjecting the scrap to a 0.125-5% solution of an alkali at 180-212 F. for a time sufiicient to loosen the cellulose nitrate layer, followed by treating the material thus obtained with a solution of potassium permanganate of at least 0.03% strength acidified with a strong mineral acid at an elevated temperature whereby the resulting product has excellent solubility characteristics.

5. A method of recovering cellulose triacetate film scrap which has thereon at least one subbing layer of cellulose nitrate, which comprises treating the scrap with 0.125% sodium hydroxide liquor for approximately 15 minutes at F., the liquid to scrap ratio being 20:1, then rinsing the material with water to free it of alkali, and then treating with water containing 0.03% of potassium permanganate and 0.06% of sulfuric acid for approximately 15 minutes at 190 F.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,099,348 Reid Nov. 16, 1937 2,337,880 Fordyce Dec. 28, 1943 2,338,672 Slack Jan. 4, 1944 2,571,686 Davis Oct. 16, 1951 

1. A METHOD OF RECLAIMING FILM SCRAP HAVING A LOWER FATTY ACID ESTER OF CELLULOSE BASE AND A CELLULOSE NITRATE SUBBING LAYER THEREON, WHICH COMPRISES SUBJECTING THE SCRAP TO AN AQUEOUS SOLUTION OF AN ALKALI OF 0.125-5% CONCENTRATION AT A TEMPERATURE OF 180-212* F. FOR A TIME SUFFICIENT TO LOOSEN THE SUBBING LAYER AND SUBSEQUENTLY TREATING THE SCRAP WITH WATER CONTAINING AT LEAST 0.03% OF AN ALKALI METAL PERMANGANATE AND 0.03-5% OF A STRONG MINERAL ACID AT AN ELEVATED TEMPERATURE WHEREBY FILM BASE MATERIAL HAVING EXCELLENT SOLUBILITY CHARACTERISTICS IS RECOVERED. 